In conventional portable/mobile data processing and communication devices, data processing and control circuitry are being packed ever more closely together with high frequency communication circuits, such as amplifiers which amplify a communication signal for transmission across a communication medium. For example, in a typical cellular telephone, the RF power amplifier is very close to the CMOS controller integrated circuit (IC) which controls the operation of the power amplifier. In an open-loop power controller arrangement, the CMOS controller delivers current to the power amplifier, and controls the delivered current appropriately to modulate the power amplifier's output as desired. The AC voltage output of the power amplifier can be 30-40 volts peak-to-peak, into 50 ohms. This means that approximately 4 amperes of AC current (for example at 2 GHz) can be inductively and resistively modulated directly onto the CMOS controller IC. This can cause AC voltages of more than 4 volts peak-to-peak to be present everywhere on the IC.
Although the output PFET of the CMOS controller IC may be able to function (with a somewhat diminished signal integrity) under these conditions, the bandgap reference circuits of the controller IC will not function under these conditions. The bipolar transistors that are used to implement diodes in conventional bandgap reference circuits exhibit minority carrier lifetimes that are typically greater than 1 nanosecond. This means that, at a frequency of 1 GHz, there is not enough time for these carriers to recombine. The impact of this is that the classical junction potential collapses and the junctions become entirely resistive in nature, so the diode junction is no longer viable as a reference.
In order to protect bandgap reference circuits from AC noise and interference, some conventional approaches use off-chip bandgap reference circuits located externally of the power amplifier controller IC. By using an off-chip bandgap reference circuit, various conventional AC noise suppression techniques such as shielding, clamping diodes and large bypass capacitors can be used to reduce the effect of AC noise and interference on the bandgap reference circuit. This approach consumes additional space and presents physical design difficulties that are not present when the bandgap reference circuit is provided on-chip, within the power controller IC.
It is therefore desirable to provide for on-chip protection of an on-chip bandgap reference circuit against AC noise/interference.